In optical devices the length of the optical paths may vary because of temperature changes in the environment. For example, the focal length of a lens system may change because the temperature change in the surrounding environment causes a change in the refractive index of the lens or other optical element. This phenomenon is particularly severe with the use of plastic or germanium lenses, and accordingly, means for compensating for changes in the focal length of an optical system due to changes in temperature has been found desirable.
A variety of devices to accomplish this purpose have been known in the prior art. Many of these have been quite complex and bulky, while others have been known to be inaccurate or unstable. Apparatus such as is found in the Leon J. Berman U.S. Pat. No. 3,612,664, which utilizes a liquid filled chamber and a piston to produce motion upon a change in ambient temperature is quite bulky and has a disadvantage of possible loss of fluid. A thermal positioning device such as shown in the W. A. Porter et al U.S. Pat. No. 2,964,903 employs a pair of parallelly mounted expansible elements, each of which has an electric coil wound thereabout to produce heat and thus linear motion of the expansible elements is found to be quite bulky, complex and unable to produce compensating motion due to ambient temperature changes of sufficient magnitude for some optical requirements. It also consumes electrical energy and requires additional means to measure temperature and adjust the voltage. An Alfred Kilgus U.S. Pat. No. 3,671,108 utilizes a compensating ring made of a material having a high coefficient of expansion and being shaped with V-shaped projections so that thermal expansion of the ring will cause linear motion of the lens which bears against the ring has been found to be unable to produce the magnitude of motion needed to compensate for large changes in focal length.